The present invention relates generally to adjustable speed drives and, more particularly, to a system and method for controlling an adjustable speed drive during generating mode of operation during deceleration using a dual loop control architecture.
Motor drives are frequently used in industrial applications to condition power and otherwise control electric driven motors such as those found with pumps, fans, compressors, cranes, paper mills, steel mills, rolling mills, elevators, machine tools, and the like. Motor drives typically provide a volts-per-hertz control and have greatly improved the efficiency and productivity of electric driven motors and applications. Increasingly, motor drives are taking the form of adjustable or variable speed drives (ASD or VSD) or adjustable frequency drives (AFD) that are adept at providing variable speed and/or variable torque control to an electric driven motor or induction machine.
If the stator terminals of an induction machine are connected to a three-phase AFD system, the rotor of the induction machine will rotate in the direction of the stator rotating magnetic field during a motoring mode of operation. When load torque is applied to the motor shaft, the steady state speed remains less than the synchronous speed. However, if the speed of the induction machine is higher than the synchronous speed when the induction machine rotates in the same direction as the stator rotating field, such as during a no load operating condition, the induction machine is in a generating mode of operation. A generating torque acting opposite the stator rotating magnetic field is produced during the generating mode, causing power to flow from the induction machine back in to the AFD.
To stop an AFD system, the AFD applies a lower frequency to the induction machine to attempt to decelerate the motor at a faster rate than if the motor were allowed to coast to a stop. During the deceleration process, the AFD continues to apply energy to the motor windings to keep the magnetic field active. Because the applied frequency is lower than the virtual frequency of the motor, the motor enters the generating mode of operation during which the generating action of the induction machine will cause the power flow to reverse the kinetic energy of the AFD system and feed power back to the power supply source. As energy is transferred from the motor to the DC link of the AFD, the DC link voltage increases and can become unstable.
One known technique for protecting the AFD during the deceleration process monitors the DC link voltage. If the DC link voltage rises above a threshold DC link voltage during deceleration, the AFD will trip and disrupt the normal stopping operation. An illustration of this technique is shown in FIG. 1. Graph 10 shows the experimental waveforms, including drive output frequency 12, DC link voltage 14, and motor current 16, for a 20 hp motor driven by a 40 hp drive from 100 Hz to 0 Hz at no load (i.e., minimum torque) at a 0.1 second deceleration rate. The top portion 18 of graph 10 is illustrated in 1 second divisions and the bottom portion 20 of graph 10 illustrates a subportion of the waveforms in 50 millisecond divisions. As the motor starts to ramp down its speed and the drive output frequency 12 decreases, the motor enters a regenerating condition and the rising DC link voltage 14 causes an overvoltage trip when the DC link voltage exceeds a threshold. After the AFD trips, the motor shuts down in an uncontrolled manner. The unstable DC link voltage and uncontrolled shutdown adds stress on the DC link capacitors, introduces EMC problems, creates undesirable harmonics and resonance, adds mechanical stress, and degrades overall system performance.
Instead of tripping, the switching control of the AFD may cause the DC link voltage to begin oscillating during the deceleration process. As an example, the graph 22 shown in FIG. 1 illustrates the captured waveforms for DC link voltage 24, motor current 26, and drive output frequency 28 for an exemplary 60 hp drive decelerating a 20 hp motor from 100 Hz to 0 Hz at no load. When the deceleration function is selected and the deceleration rate is set as 0.1 seconds, the motor begins ramping down its speed into a regenerating condition. As shown in graph 22, the DC link voltage 24 becomes oscillatory with an overshoot exceeding 150V. That is, at certain frequencies, the voltage will jump between high and low values and disrupt the normal deceleration process, resulting in EMI and EMC interferences with surrounding equipment.
Another known solution for controlling the deceleration process employs a braking resistor, which provides a path to dissipate the regenerative energy. The braking resistor control circuit senses the high DC voltage condition and electrically connects the braking resistors across the DC link. While the braking resistors can be effective in dissipating excess energy, the costs of the braking resistor can be significant. Also, the large physical size of the braking resistor significantly increases the overall size of the AFD.
It would therefore be desirable to provide a system and method for controlling an AFD during a generating mode of operation that maintains a smooth DC link voltage during deceleration without tripping the AFD or generating an oscillation in the DC link voltage. It would further be desirable to provide a system and method for controlling an AFD without a braking resistor to minimize the size and cost of the AFD.